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One, called solanezumab, has been shown to slow mental decline in people with mild-to-moderate disease by a third.

But results were less positive than hoped, and researchers are increasingly convinced that the best way to attack Alzheimer’s is to stop the underlying disease before the plaques form in large numbers.

As a result they have started to look at agents which tackle the key ingredient of the plaques, a protein called beta amyloid.

In a pilot study of 200 healthy volunteers, drugs firm MSD showed that its agent MK-8931 reduced levels of beta amyloid in spinal fluid by 92 per cent. Results were presented to the American Academy of Neurology earlier this year.

Now it is rolling out the study to 1,700 people with mild-to-moderate Alzheimer’s worldwide. Half will be randomly assigned the drug and half a placebo. Some of the study participants will be in Britain, although numbers have yet to be finalised.

Dr Richard Perry, a consultant neurologist at lecturer at Imperial College Healthcare NHS Trust, said: “The idea of this drug is to stop the production of abnormal levels of beta amyloid in the brain.

"It’s about getting in early, so that if less amyloid is produced less plaques will come together.”

He continued: “From what I have seen of the phase one trial results, this drug looks encouraging in terms of reducing the level of abnormal beta-amyloid in spinal fluid.

“It’s demonstrating that it ‘does what is says on the tin’.”

But he also urged caution: “The big question with all these medications, is ‘Does that translate to having a beneficial effect in real life, in terms of daily activities?’ “

The research scientists will measure rates of cognitive decline in those on the pill against those on the dummy, using mental ability tests and assessing how well they carry out normal day-to-day functions.

They are not expecting it to improve the abilities of sufferers: most experts think that there is no way of reversing the damage Alzheimer's has already inflicted.

Because they are looking at relatively early-stage Alzheimer’s, the differences will take some time to accrue: the researchers say results will not be ready until 2016.

Other drugs that tackle Alzheimer’s early in the course of the disease are also in the pipeline. Roche, Eisei and Lilly all have trials planned.

Dr William Thies, chief medical officer of the Alzheimer’s Association in the US, was optimistic that one would bear fruit soon.

He said: “The number of smart people working on this problem means to me we'll begin to manage it better in the very near future. It may be as short as three years away."

Jessica Smith, research officer at the Alzheimer’s Society, said: "It’s interesting to see that this research is taking a different approach to much of the previous research that has been conducted and we are really interested to see what the results show."

Alzheimer's is the most common form of dementia, affection about 500,000 people in Britain.